I would like to understand which mechanism triggers the first cell differentiation after n divisions.

I read some articles (on Biology Stack Exchange and Wikipedia) on cellular differentiation and embryogenesis but still fail to understand what exactly makes it so that starting from a given division cells suddenly start to be different.

Wikipedia claims that

In the first hours after fertilization, this cell divides into identical cells. In humans, approximately four days after fertilization and after several cycles of cell division, these cells begin to specialize, forming a hollow sphere of cells, called a blastocyst

but do not explain why they begin to specialize.

I could imagine

that a cell has a "counter" on the number of divisions which triggers differentiation after a given amount of divisions

or a chemical substance (either cell-borne or external) forces a change in the division

but why some cells would become "cell A" and some others "cell B"?

I am fairly sure that the biochemical mechanisms which regulate the life of a grown up mechanism can explain cellular differentiation (through hormones for instance) -- I am however interested by this specific moment, this n-th division where identical cells become differentiated.

The cues comes from the combination of one or more factors, same cell, adjoining cells same or different and environment.After 'n' divisons at a time 't', the proteins responsible for cell specificaiton are produced later in the cell cylce after sufficient number of replication. the factors that starts specificaiton comes from,

1. neibhouring cells 2. factors from neibhouring cells suppress the replication factor but increases the specificaiton factor3. replication factor is retained in the parent cell but daughter cell is lack of replication factor and starts specificaiton4. the site "niche" for stem cells maintains the replication of adult stem cells but once the cells moves away, specificaiton starts5. "reciprocal induction" where two neibhouring cells determine each others fate

Hi, i think the mechanism that directs the initial stages of differentiation is inherent in the egg before fertilization. The egg carries certain proteins and mRNAs in sufficient amount that regulates the initial stages of development till the embryo is ready to carry out its own regulations. Other factors that may contribute are environmental factors like centre of gravity, direction of physical forces etc.

"And he who wants to be a creator in good and evil has first to be a destroyer and breakvalues."N

Hi, i think the mechanism that directs the initial stages of differentiation is inherent in the egg before fertilization. The egg carries certain proteins and mRNAs in sufficient amount that regulates the initial stages of development till the embryo is ready to carry out its own regulations. Other factors that may contribute are environmental factors like centre of gravity, direction of physical forces etc.

"And he who wants to be a creator in good and evil has first to be a destroyer and breakvalues."N

As far as i know , the cell differentiation is close related with two factors: time and location .As you know ,the differentiation began since the fertilization , later contact with neighboring cell can also lead to farther differentiation. I think the first differentiation is decided by gene , the konwledge of gene and epigenetics may help you .